Percutaneous transluminal coronary angioplasty (PTCA) is widely accepted as an effective treatment of blockages in the coronary arteries. Blockages (stenoses) may occur from cholesterol precipitation on the coronary wall which may be in any stage from initial deposit through aged lesions. Coronary arteries can also become blocked due to formation of thrombus.
The most widely used percutaneous coronary angioplasty makes use of a dilatation balloon catheter. The catheter is inserted into the patient's vascular system and guided until the balloon at the distal end of the catheter is positioned across the stenosis. A radiographic contrast fluid is then fed under pressure through an inflation lumen of the catheter to the balloon, which causes the balloon to expand outward, thereby opening the stenosis.
Various types and configurations of dilatation balloon catheters have been known and used in the prior art. Examples are shown, for example, in U.S. Pat. No. 5,040,548 to Yock, U.S. Pat. No. 5,061,273 to Yock, and in U.S. Pat. No. 4,762,129 to Bonzel (see also Reexamination Certificate No. B14,762,129).
One important characteristic of a dilatation balloon catheter used for angioplasty is its profile, i.e., the outer diameter of its distal end portion when deflated. Considerable effort has been spent in developing low-profile dilatation balloon catheters by minimizing the dimensions of the core or inner tube which extends through the balloon to its distal end, and by reducing wall thickness, to the extent possible, of the balloon itself.
The outer diameter of the deflated distal end portion of a balloon dilatation catheter affects the ease and ability of the dilatation catheter to pass through a guide catheter, through the coronary arteries, and across tight lesions. Application of low-profile balloons can be in a variety of environments, including, but not limited to, over-the-wire, fixed-wire, and monorail systems, as well as with guiding catheters.
A complicating factor in minimizing the deflated profile of a dilatation catheter balloon is that the balloon membrane is typically not distensible, i.e., it does not stretch or contract in response to changes in internal pressure. Thus, the balloon membrane has a constant surface area regardless of whether the balloon is inflated or deflated. Therefore, in order to reduce the outer diameter of the balloon in its deflated condition, it is common to fold the balloon flat, so that two wings or flaps are formed. These two wings are then brought together in some fashion, as by folding or wrapping, so as to reduce the overall diameter of the deflated balloon. Often, some sort of protective sleeve or sheath is disposed around the folded or wrapped balloon to protect the balloon from contamination or damage prior to its use.
In actual use, when inflation fluid is applied to the folded balloon, it causes the flaps to unwrap so that the balloon can inflate to its full inflated state.
While it is desirable to minimize profile, it is also desirable to provide as large as possible an inflated outer diameter of the balloon relative to the deflated profile. One practical effect is that the two flaps formed when the balloon is deflated and prepared for wrapping (during balloon protector installation) become very large relative to the core or inner tube of the catheter. The result is that it is difficult to get these two large flaps to fold together and squeeze out all of the space between them when folded, without damaging the catheter during balloon protector installation.
Various methods and balloon configurations have been proposed in the prior art for providing a dilatation balloon catheter having the lowest profile as possible when deflated and the largest possible diameter when inflated. One approach, which is suggested, for example, in U.S. Pat. No. 5,087,246 to Smith and in U.S. Pat. No. 5,147,302 to Euteneuer et at., is to provide a dilatation balloon having more than two flaps or wings, (for example, three wings) such that when the flaps or wings are wrapped circumferentially, the distance that each flap extends around the catheter is reduced compared with the conventional balloon configuration having only two flaps. The ease with which such flaps fold is also enhanced when their number is increased, such that when the balloon is deflated and withdrawn through the guide catheter following a procedure, the balloon more readily returns to its wrapped condition. The result is a reduced deflated profile given the same inflated diameter.
The above-referenced U.S. Pat. No. 5,147,302 to Euteneuer et al. proposes two different methods for formation of a tri-fold dilatation balloon. In one method, a clamping fixture is used to clamp approximately one-third of the distance across the balloon, this clamped portion defining a first wing or flap. Then the balloon is inflated at low pressure such that the unclamped portion of the balloon is inflated. Finally, pressure is applied against the exterior of the balloon while the balloon is deflated, so that the unclamped portion of the balloon is pressed against the side of the clamp, forming the second and third wings of the balloon.
Another method proposed in the Euteneuer et al. '302 patent involves centering the balloon within a tubular fixture having radially retractable blades circumferentially spaced at 120.degree. intervals. Once the balloon is positioned, the blades are simultaneously moved inward toward the core of the balloon, while a vacuum is applied to the balloon.
Of course, a dilatation balloon must be deflated prior to withdrawal of the dilatation catheter from the patient's vascular system through the guiding catheter used in an angioplasty procedure. It is thus important that the balloon be reliably collapsible to its minimal, radially compact profile. Balloons having only two flaps or wings have proven to be fairly reliable in this regard. Applying negative pressure to the inflation lumen of the catheter causes the balloon to flatten, reforming the two wings. However, as the number of flaps or wings is increased, it becomes more difficult to ensure symmetrical deflation of the balloon.
One method that has been proposed in the prior art for enhancing a balloon's ability to collapse symmetrically is to subject the balloon to heat-treatment when it is initially brought into a multi-fold configuration and wrapped. Such heat-setting approaches have been suggested, for example, in the above-referenced Euteneuer et al. '302 patent and in the above-referenced Smith '246 patent.
While the foregoing may represent some improvement in field of balloon dilatation catheters, the inventor believes that there is an ongoing need for improvements in catheter design and preparation techniques, such that low (deflated) profile and large inflated balloon diameters may be achieved without sacrificing other characteristics, such as reliability of symmetrical deflation.